Lymphatic malformations (LMs) result from improper formation of lymphatic vessels. The goal of the lymphatic vessels is to recover the body's tissue fluid, which may pass through lymph nodes, and to return it to the bloodstream. When the vessels do not properly form, the transfer slows or stops, and masses (malformations) may develop as a result of fluid accumulation. The masses may resemble sponge-like solid tissue (microcystic), or include cystic spaces of variable size (macrocystic). Lymphatic malformations can also result in abnormal accumulation of lymphatic fluid in body cavities (pleural effusions in the chest, ascites in the abdomen).
It is believed that lymphatic malformations form during embryonic or fetal development. Nothing a pregnant mother does is known to cause or prevent LM formation.